Fixed type constant velocity universal joint

ABSTRACT

A fixed type constant velocity universal joint comprising an outer joint member ( 1 ) having eight arcuate guide grooves ( 1   b ) extending in the axial direction in an inner spherical surface ( 1   a ), an inner joint member ( 2 ) having eight arcuate guide grooves ( 2   b ) extending in the axial direction in an outer spherical surface ( 2   a ), eight balls ( 3 ) disposed between the guide grooves ( 1   b ) of the outer joint member ( 1 ) and guide grooves ( 2   b ) of the inner joint member ( 2 ), and a cage ( 4 ) interposed between the outer and inner joint members ( 1 ) and ( 2 ) for retaining the balls ( 3 ), in which center (P) of guide grooves ( 1   b ) of the outer joint member ( 1 ) is offset from center (O) of inner spherical surface ( 1   a ), and center (Q) of guide grooves ( 2   b ) of the inner joint member ( 2 ) is offset from center (O) of outer spherical surface ( 2   a ) by equal distance (PO=QO=F) to opposite sides in the axial direction, and the relation of 0.45≦B/A≦0.65 is established, where A stands for the wall thickness of the cage ( 4 ), and B the radial displacement of balls  3  at maximum joint angle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a constant velocity universal joint used in power transmission in automobiles and various industrial machines, and more particularly to a fixed type constant velocity universal joint having eight balls.

2. Prior Art

Constant velocity universal joints are classified into a fixed type not sliding in the axial direction, and a slidable type. FIGS. 8 a and 8 b show a Rzeppa type constant velocity universal joint (hereinafter called BJ type) as a representative example of fixed type constant velocity universal joint. This constant velocity universal joint comprises an outer ring 11 as an outer joint member having six arcuate guide grooves 11 b extending in the axial direction in an inner spherical surface 11 a, an inner ring 12 as an inner joint member having six arcuate guide grooves 12 b extending in the axial direction in an outer spherical surface 12 a, six balls 13 disposed between the guide grooves 11 b of the outer ring 11 and guide grooves 12 b of the inner ring 12, and a cage 14 for retaining the balls 13.

Centers P and Q of the guide grooves 11 b and 12 b are offset from the joint center O by an equal distance (PO=PQ) to the right and left side in the axial direction. That is, the center P of guide groove 11 b of the outer ring 11 is offset from center O of the inner spherical surface 11 a to the opening side of the outer ring 11 by distance PO. The center Q of guide groove 12 b of the inner ring 12 is offset from center O of the outer spherical surface 12 a to the inner side of the outer ring 11 by distance QO. The centers of inner spherical surface 11 a of the outer ring 11 and outer spherical surface 12 a of the inner ring 12 coincide with the joint center O.

One (not shown) of two shafts to be coupled is connected to the outer ring 11, and other (shaft part 15) is connected to the inner ring 12. Accordingly, the inner ring 12 has a tooth profile, i.e. serration or spline, 12 c to be coupled with the shaft part 15. The outer ring 11 and inner ring 12 form a certain angle, and the balls 13 guided in the cage 14 are maintained within a plane perpendicular to a bisector of angle θ formed by the outer and inner ring 11 and 12, so that the constant velocity of the joint is assured.

In a fixed type constant velocity universal joint, hitherto, a constant velocity universal joint applicable to high angle (maximum joint angle 50°) was an undercut-free joint (called UJ type) comprising six balls 13, but by increasing the number of balls and reducing the ball diameter, a more compact UJ type having eight balls with same strength and durability has been developed (for example, see Japanese Patent Application laid-open under No. H9-317784).

SUMMARY OF THE INVENTION

In a fixed type constant velocity universal joint, when torque is transmitted in a state of forming a working angle, in the cage pockets, the balls move in the circumferential direction, and also move in the radial direction at the same time (see FIG. 1). In the conventional fixed type constant velocity universal joint of six balls, the wall thickness of the cage has been determined to set the minimum limit to satisfy the displacement of balls in the radial direction at the maximum joint angle, in other words, so that the ball contact point may settle within the cage pocket. This is because increase of wall thickness of the cage causes the depth of guide grooves in the outer and inner rings to be reduced, thereby lowering the durability life of the joint at high angle and high load.

In the fixed type constant velocity universal joint of eight torque transmission balls realizing a more compact design and lighter weight, it is important that the strength at high joint angle should be kept same as that of the conventional constant velocity universal joint of six balls. To increase the strength of the cage, it is easy to increasing the wall thickness of the cage, but by increasing the thickness of the cage, the depth of the guide grooves of the outer and inner rings becomes shallower. As the guide grooves of the outer and inner rings become shallow, contact ellipses of balls can ride over the guide grooves at high angle and high torque load, and the durability life is lowered.

It is hence an object of the invention to present a fixed type constant velocity universal joint of eight balls capable of satisfying two important functions, that is, assurance of cage strength at high angle and assurance of durability life at high angle and high torque load.

It is other object of the invention to present a fixed type constant velocity universal joint of eight balls seeking the optimum thickness of cage in order to have the cage strength at high angle and durability life at high angle and high load equivalent to that of the conventional fixed type constant velocity universal joint of six balls.

It is one aspect of the invention to present a fixed type constant velocity universal joint comprising an outer joint member having eight arcuate guide grooves extending in the axial direction in an inner spherical surface, an inner joint member having eight arcuate guide grooves extending in the axial direction in an outer spherical surface, eight balls disposed between the guide grooves of the outer joint member and guide grooves of the inner joint member, and a cage interposed between the outer joint member and inner joint member for retaining the balls, in which center of guide grooves of the outer joint member is offset from center of inner spherical surface, and center of guide grooves of the inner joint member is offset from center of outer spherical surface by equal distance to opposite sides in the axial direction, and the relation of 0.45≦B/A≦0.65 is established, where A stands for the wall thickness of the cage, and B the radial displacement of balls at maximum joint angle.

It is other aspect of the invention to present a fixed type constant velocity universal joint comprising an outer joint member having eight arcuate guide grooves extending in the axial direction in an inner spherical surface, an inner joint member having eight arcuate guide grooves extending in the axial direction in an outer spherical surface, eight balls disposed between the guide grooves of the outer joint member and guide grooves of the inner joint member, and a cage interposed between the outer joint member and inner joint member for retaining the balls, in which center of guide grooves of the outer joint member is offset from center of inner spherical surface, and center of guide grooves of the inner joint member from center of outer spherical surface, by equal distance to opposite sides in the axial direction, and inner spherical surface of the cage is offset from outer spherical surface thereof to the opposite side by equal distance in the axial direction, and further straight portions having straight groove bottoms are provided in guide grooves of the outer joint member and inner joint member, and the relation of 0.65≦B/A≦0.85 is established, where A stans for the wall thickness of the cage, and B the radial displacement of balls at maximum joint angle.

Value R1 of ratio (F/PCR) of offset amount F to length PCR of line segment linking center of guide grooves of the outer joint member or center of guide grooves of the inner joint member and center of balls may be specified in a range of 0.069≦R1≦0.121.

Contact angle α of guide grooves and balls may preferably be in a range of 29° to 40°. When the offset amount F is decreased, the inversion start angle of wedge angle between guide grooves of the outer joint member and inner joint member becomes smaller, and when the wedge angle is inverted, the play of the balls in the guide grooves is increased, and hammering sound can be generated. Accordingly, to set the wedge angle inversion start angle at least more than the ordinary angle of vehicle or the like, the contact angle α of guide grooves and balls is set in a range 29° to 40°. Herein, the wedge angle refers to the angle formed by a common normal of load side contact point of balls and guide grooves of outer joint member, and a common normal of load side contact point of balls and guide grooves of inner joint member. The ordinary angle of vehicle is generally 9° or less. As the joint angle increases, the wedge angle comes to the minus side going down below zero. This change of sign of wedge angle below zero is called inversion of wedge angle.

The following table shows the relation of the value R1 of ratio (F/PCR) of offset amount F to length PCR of line segment linking center of guide grooves of the outer joint member or center of guide grooves of the inner joint member and center of balls, contact angle α, and wedge angle inversion start angle. TABLE 1 Value of Inversion start angle ratio F/PCR Contact angle α (°) of wedge angle (°) 0.069 45 8 40 9 29 14 0.121 45 14 40 16 29 25

As known from Table 1, the smaller the contact angle α, the larger becomes the wedge angle inversion start angle. Also, the larger the offset amount F, the larger becomes the wedge angle inversion start angle. Therefore, as in the configuration described above, by setting the value R1 of ratio (F/PCR) of offset amount F to length PCR of line segment linking center of guide grooves of the outer joint member or center of guide grooves of the inner joint member and center of balls in a range of 0.069≦R1≦0.121, and specifying the contact angle α of guide grooves and balls in a range of 29 degrees to 40 degrees, the contact ellipse of balls will not be dislocated from the guide grooves to ride over the spherical surface, and at least at the joint angle smaller than the ordinary joint angle of vehicle, play of balls in the guide grooves is eliminated, generation of hammering sound is prevented.

The fixed type constant velocity universal joint of the invention assures the specified thickness of the cage, and realizes a more lightweight and compact structure while having the durability life and high angle strength equivalent to that of the conventional fixed type constant velocity universal joint of six balls.

Referring now to the drawings, preferred embodiments of the invention will be described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a longitudinal sectional view of a joint for explaining the structure of the invention;

FIG. 1 b is a sectional view of a cage;

FIG. 2 a is a longitudinal sectional view of joint according to an embodiment of the invention;

FIG. 2 b is a cross-sectional view of the joint shown in FIG. 2 a;

FIG. 3 is a partial, magnified view of FIG. 2 a;

FIG. 4 is a partial, magnified view of FIG. 2 b;

FIG. 5 is a partial, magnified view of FIG. 3;

FIG. 6 is a longitudinal sectional view of a joint according to other embodiment;

FIG. 7 is a diagram showing the relation of cage strength and allowable load torque in terms of wall thickness of cage;

FIG. 8 a is a longitudinal sectional view of a conventional fixed type constant velocity universal joint; and

FIG. 8 b is a cross-sectional view of the joint shown in FIG. 8 a.

DESCRIPTION OF PREFERRED EMBODIMENTS

First, an embodiment of BJ type shown in FIGS. 2 a, 2 b and FIG. 3 is explained. FIG. 2 a is a longitudinal sectional view of a joint, and FIG. 2 b is a cross-sectional view of the joint. FIG. 3 is a partial, magnified view of FIG. 2 a.

As shown in FIGS. 2 a, 2 b and 3, the fixed type constant velocity universal joint is mainly composed of an outer ring 1, an inner ring 2, balls 3, and a cage 4. The outer ring 1 as outer joint member is formed like a cup, and a shaft part for connecting with one of two shafts to be coupled is formed at the closed end side thereof. The outer ring 1 has a spherical inner circumference, that is, an inner spherical surface 1 a, and eight arcuate guide grooves 1 b extending in the axial direction are formed in the inner spherical surface 1 a. The inner ring 2 as inner joint member has a tooth profile, i.e. serration or spline, 2 d to be connected with other one (shaft part 5) of two shafts to be coupled. The inner ring 2 has a spherical outer circumference, that is, an outer spherical surface 2 a, and eight arcuate guide grooves 2 b extending in the axial direction are formed in the outer spherical surface 2 a. The guide grooves 1 b of the outer ring 1 and guide grooves 2 b of the inner ring 2 form pairs, and one ball is disposed in a ball track formed by each pair of guide grooves 1 b and 2 b. A total of eight balls 3 are retained at equal intervals in the circumferential direction by the cage 4. The cage 4 has concentric outer and inner spherical surfaces 4 a and 4 b, and the outer spherical surface 4 a spherically contacts the inner spherical surface 1 a of the outer ring 1, and the inner spherical surface 4 b spherically contacts the outer spherical surface 2 a of the inner ring 2.

In this embodiment, centers P and Q of the guide grooves 1 b and 2 b are respectively offset from joint center O by equal distance (PO=QO=F) to the right and left side in the axial direction. That is, the center (outer ring track center) P of guide grooves 1 b of the outer ring 1 is offset from the center O of the inner spherical surface 1 a to the opening side of the outer ring 1 by distance PO. The center (inner ring track center) Q of guide grooves 2 b of the inner ring 2 is offset from the center O of the outer spherical surface 2 a to the inner side of the outer ring 1 by distance QO. The center of the outer spherical surface 4 a of the cage 4, and the center of the inner spherical surface 1 a of the outer ring 1 as the guide surface of the outer spherical surface 4 a of the cage 4 both coincide with the joint center O. Similarly, the center of the inner spherical surface 4 b of the cage 4, and the center of the outer spherical surface 2 a of the inner ring 1 as the guide surface of the inner spherical surface 4 b of the cage 4 both coincide with the joint center O. Therefore, the offset amount (PO=F) of the outer ring 1 is the axial distance between the center P of the guide grooves 1 b and the joint center O, and the offset amount (QO=F) of the inner ring 2 is the axial distance between the center Q of the guide grooves 2 b and the joint center O, and the both are equal to each other.

Length PO₃ of line segment linking center P of guide grooves 1 b of outer ring 1 and center O₃ of ball 3, and length QO₃ of line segment linking center Q of guide groove 2 b of inner ring 2 and center O₃ of balls 3 are equal to each other, as indicated by PCR in FIG. 2 a. Further, as shown in FIG. 3, an angle formed between the line segment linking center P of guide grooves 1 b of outer ring 1 and center O₃ of ball 3, and the line segment linking joint center O and center O₃ of balls 3 is called outer ring track offset angle βo, and an angle formed between the line segment linking center Q of guide grooves 2 b of inner ring 2 and center O₃ of ball 3, and the line segment linking joint center O and center O₃ of balls 3 is called inner ring track offset angle βi, and the outer ring track offset angle βo and inner ring track β1 are equal to each other.

In this configuration, one (not shown) of two shafts to be coupled and outer ring 1 are connected, and other (shaft part 5) and inner ring 2 are connected. When the outer ring 1 and inner ring 2 form a certain angle, the balls 3 guided in the cage 4 are maintained within a plane perpendicular to a bisector of angle θ formed by the outer and inner rings 1 and 2, and therefore the distances PO₃ and QO₃ from the ball center O₃ to the guide groove centers P and Q are equal to each other (PO₃=QO₃=PCR), so that the constant velocity of the joint is assured.

As described above, the offset amount (F=PO=QO) of the guide grooves 1 b and 2 b is set in range of value R1 of ratio F/PCR of 0.069≦R1≦0.121, and it is preferred from the viewpoint of assuring the allowable load torque, assuring the cage strength, assuring the durability, and assuring the working efficiency, and in this embodiment, the ratio is set at R1=0.104 (or 0.1038). In a comparative example (fixed type constant velocity universal joint of six balls as shown in FIG. 8), a general value of R1 is 0.14, and R1 of the embodiment is considerably smaller than in the comparative example.

FIG. 4 is a partial, magnified view of FIG. 2 b, showing a mutual relation of outer ring 1, inner ring 2, and balls 3. Guide grooves 1 b formed in the inner spherical surface 1 a of the outer ring 1 have a Gothic arch cross section, and guide grooves 2 b formed in the outer spherical surface 2 a of the inner ring 2 have also a Gothic arch cross section. Therefore, the balls 3 contact the guide grooves 1 b of the outer ring 1 at two points C₁₁ and C₁₂, and contact guide grooves 2 b of the inner ring 2 at two points C₂₁ and C₂₂. Angle α formed by center O₃ of balls 3 corresponding to line segment passing the center O₃ of balls 3 and joint center O and contact points C₁₁, C₁₂, C₂₁, C₂₂ with guide grooves 1 b and 2 b is the contact angle. Contact angles α of contact points C₁₁, C₁₂, C₂₁, C₂₂ are all equal to each other, and set in a range of 29° to 40°. The contact angle α of 29° to 49° is smaller as compared with 37° to 45° in the conventional undercut-free joint of six balls, fixed type joint of six balls or fixed joint of eight balls. By setting the contact angle α at 29° or more, the contact surface pressure of the guide grooves and balls can be suppressed, and the durability equivalent to the prior art can be obtained.

FIG. 5 is a magnified view in part of FIG. 3 for explaining the inversion start angle of wedge angle. As mentioned above, the angle formed by common normal H₁ of contact point C₁ of balls 3 and guide grooves 1 b of outer ring 1, and a common normal H₂ of contact point C₂ of balls 3 and guide grooves 2 b of inner ring 2 is called the wedge angle 2τ. The common normal H₁ is a three-dimensional straight line linking the contact point of outer ring 1 and balls 3 and ball center O₃. Similarly the common normal H₂ is a three-dimensional straight line linking the contact point of inner ring 2 and balls 3 and ball center O₃. As shown in FIG. 5, contact point C₁ of guide grooves 1 b of outer ring 1 and balls 3 is inclined to the joint center surface passing through center O₃ of balls 3 by angle τ owing to the arcuate guide grooves 1 b. Similarly, contact point C₂ of guide grooves 2 b of inner ring 2 and balls 3 is inclined to the joint center plane passing through center O₃ of balls 3 by angle τ owing to the arcuate guide grooves 2 b. The wedge angle is equal to the sum 2τ of these wedge angles τ. The wedge angle 2τ decreases in a certain phase along with increase of joint angle, and is then inverted. The joint angle upon start of inversion of wedge angle 2τ is set at 9° or higher.

FIG. 6 shows an embodiment of UJ type. This embodiment is same as the embodiment in FIG. 1 except that a straight portion 1 c is provided in the guide grooves 1 b of the outer ring 1 c, that a straight portion 1 c is provided in the guide grooves 2 b of the inner ring 2, and that the spherical centers p and q of the outer spherical surface 4 a and inner spherical surface 4 b of the cage 4 are offset in the opposite directions by equal distance f in the axial direction.

Referring back to FIGS. 1 a and 1 b, in the fixed type constant velocity universal joint of eight balls, the cage strength at high angle and durability life at high angle and high load are compared with the conventional fixed type constant velocity universal joint of six balls, and the optimum value of the cage wall thickness for achieving the equivalent performance is explained. In FIG. 1 a, the joint in FIG. 2 a has the maximum joint angle θmax. The ball appearing at the upside in the drawing is indicated by reference numeral 3 a, and the phase of this ball 3 a is the phase of the ball coming to the innermost side of the cage. The ball appearing at the downside in the drawing is indicated by reference numeral 3 b, and the phase of this ball 3 b is the phase of the ball coming to the outermost side of the cage. FIG. 1 b shows the balls 3 a and 3 b by double dot chain line in the longitudinal sectional view of the cage 4, in which the distance between centers of ball 3 a and ball 3 b indicated by character B is the radial displacement of the ball in the pocket 4 c of the cage 4. The wall thickness of the cage 4 is the radial dimension of the peripheral wall of the pocket 4 c, and it is indicated by character A in FIG. 1 b.

Supposing the wall thickness of the cage 4 to be A and the radial displacement of the ball 3 in one revolution of the joint to be B, the wall thickness A of the cage 4 is defined to satisfy the range of 0.45≦B/A≦0.65 in the BJ type, and satisfy the range of 0.65≦B/A≦0.85 in the UJ type. The optimum value range of wall thickness of cage differs between the BJ type and UJ type because, as already explained in relation to FIG. 6, the UJ type has straight portions 1 c and 2 c in the guide grooves 1 b and 2 b of the outer and inner rings, and the guide grooves are shallower at the joint inner side as compared with the BJ type.

The meaning of this numerical value range is illustrated in FIG. 7. In FIG. 7, the numerical values refer to the BJ type, but a similar tendency is noted in the UJ type, too. In the diagram, T100 torque is a basic torque used in life calculation of constant velocity universal joint, and is the torque value determined from the contact stress (Hertz stress) of the guide grooves and balls of the outer ring and inner ring, and it means the torque capable of obtaining a life of 1500 hours at 100 rpm. As shown in FIG. 7, at B/A<0.45, in other words, when the cage wall thickness A is larger more than necessary than the radial displacement B of the balls 3 in the pocket 4 c of the cage 4, a sufficient strength of the cage 4 is assured, but the guide groove depth (inner side) of the inner and outer rings is too shallow, and the contact ellipse of the balls 3 can go out of the guide grooves at maximum joint angle, and the load torque is lowered extremely, and the joint function can be lost. On the other hand, at B/A>0.65, in other words, when the cage wall thickness A is not sufficient as compared with the radial displacement B of the balls 3 in the pocket 4 c of the cage 4, the guide groove depth (inner side) of the inner and outer rings is sufficient, and the contact ellipse of the balls 3 will not go out of the guide grooves. To the contrary, the wall thickness A of the cage 4 is not enough, and the cage strength at high angle cannot be assured. In the UJ type having eight balls, a similar tendency as in the BJ type is noted. Since the structure is thus different, the numerical value range is also different.

Thus, in order to satisfy the both important properties of cage strength at high angle and joint durability at high angle, an optimum wall thickness setting of the cage is required, and the range is as specified above (0.45≦B/A≦0.65 in the BJ type, and 0.65≦B/A≦0.85 in the UJ type). 

1. A fixed type constant velocity universal joint comprising: an outer joint member having eight arcuate guide grooves extending in the axial direction in an inner spherical surface thereof, an inner joint member having eight arcuate guide grooves extending in the axial direction in an outer spherical surface thereof, eight balls disposed between the guide grooves of the outer joint member and guide grooves of the inner joint member, and a cage interposed between the outer joint member and inner joint member for retaining the balls, wherein center of guide grooves of the outer joint member is offset from center of inner spherical surface, and center of guide grooves of the inner joint member is offset from center of outer spherical surface by equal distance (F) to opposite sides in the axial direction, and wherein the relation of 0.4523 B/A≦0.65 is established, where A stands for the wall thickness of the cage, and B the radial displacement of balls at maximum joint angle.
 2. A fixed type constant velocity universal joint comprising: an outer joint member having eight arcuate guide grooves extending in the axial direction in an inner spherical surface thereof, an inner joint member having eight arcuate guide grooves extending in the axial direction in an outer spherical surface thereof, eight balls disposed between the guide grooves of the outer joint member and guide grooves of the inner joint member, and a cage interposed between the outer joint member and inner joint member for retaining the balls, wherein center of guide grooves of the outer joint member is offset from center of inner spherical surface, and center of guide grooves of the inner joint member is offset from center of outer spherical surface by equal distance (F) to opposite sides in the axial direction, and inner spherical surface of the cage is offset from outer spherical surface thereof to the opposite side by equal distance (f) in the axial direction, wherein straight portions having straight groove bottoms are provided in guide grooves of the outer joint member and inner joint member, and wherein the relation of 0.65≦B/A≦0.85 is established, where A stands for the wall thickness of the cage, and B the radial displacement of balls at maximum joint angle.
 3. The fixed type constant velocity universal joint of claim 1, wherein value R1 of ratio (F/PCR) of offset amount (F) to length (PCR) of line segment linking center of guide grooves of the outer joint member or center of guide grooves of the inner joint member and center of balls is specified in a range of 0.069≦R1≦0.121.
 4. The fixed type constant velocity universal joint of claim 2, wherein value R1 of ratio (F/PCR) of offset amount (F) to length (PCR) of line segment linking center of guide grooves of the outer joint member or center of guide grooves of the inner joint member and center of balls is specified in a range of 0.069≦R1≦0.121.
 5. The fixed type constant velocity universal joint of claim 1, wherein contact angle of guide grooves and balls is in a range of 29° to 40°.
 6. The fixed type constant velocity universal joint of claim 2, wherein contact angle of guide grooves and balls is in a range of 29° to 40°. 